Passive intermodulation (PIM) causes an unwanted signal or signals to be generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that can act as sources of PIMs that interfere with system operation.
PIM signals are created when two signals from different systems combine and are then reflected at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. Generally the components of concern are third, fifth, and seventh order, where the third order is of greatest signal strength, and therefore, of primary concern. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems are utilized to detect PIMs enabling a technician to locate the faulty cable or connector. The test system to measure PIMs creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected and the PIM and distance to PIM measurement is provided to an operator.
FIG. 1 is a block diagram of a prior art test system setup for measuring PIM. The test system utilizes two signal sources, with a first signal source 2 producing a signal at frequency F1 and the second signal source 4 producing a signal at frequency F2. When these multiple signals are allowed to share the same signal path in a nonlinear transmission medium, the unwanted signals can occur. The third order response is particularly troublesome as it produces signals at 2F1-F2 as well as 2F2-F1. Test signals F1, F2 generated by the signal sources 2, 4 are provided to a combiner 6 to create a combined signal with frequencies F1 and F2 at the combiner output. A diplexer 8 sends the combined signal F1 and F2 to a test port 10 connected with a load and a PIM source 30. A reverse or reflected signal from port 10 is then produced at frequency 2F1-F2, and forwarded through the diplexer 8 to be down converted to an intermediate frequency. The reverse or reflected signal is output to a mixer 12, 22 where it is combined with a signal LO1, LO2 generated by a local oscillator (LO) 14, 24, and the target frequency or frequencies is selected by filtering the mixer output using a low-pass filter 16, 26. As shown, the reverse or reflected signal is down converted to a target intermediate frequency in two stages. The magnitude of the intermediate frequency signal is detected by a receiver 32 and the PIM measurement 34 is obtained.
FIG. 2 is a simplified block diagram, again showing a test signal including two or more test signal frequencies provided to a diplexer 8 that sends the test signal to a test port 10 connected with a load and a PIM source 30. However, the block diagram illustrates a further source of PIM (i.e. residual PIM) 40 within the PIM measuring instrument. There are multiple internal mechanisms in a PIM measuring instrument that can degrade over time including the test port connector, the diplexing filter, or internal crosstalk between output power amplifier sources. The presence of residual PIM can corrupt the measurement of the device being measured. Once this residual PIM reaches a limit approaching the magnitude of the PIM under test, the system becomes unusable and must be sent back to the factory for repair. It would be desirable to reduce or eliminate the influence of residual PIM on the overall PIM measurement output by the receiver.